The invention pertains to a process for producing an elongated component of glass by drawing from a blank, in which the blank is sent to a heating zone, where it is softened region by region; where the component is drawn off continuously in a controlled manner from the softened region, which forms a "drawing bulb"; where estimates of at least one controlled variable are predicted without any dead time; and where, furthermore, at least one geometric variable of the component which can be correlated with the controlled variable is measured continuously, the measurement values thus obtained being used to adjust the predicted estimates; and where the deviation between the nominal value and the actual value of the controlled variable is determined on the basis of the predicted estimates which have been adjusted in this way, the deviation then being converted to a change in a controlling variable.
Methods such as this are suitable for the production of, for example, tubes, rods, hollow fibers, and solid fibers.
The geometry of the component plays a decisive role in many applications where these components are used. Hollow fibers, for example, are used in chromatography, and tubes serve as semi-finished material for the production of preforms for optical waveguides or even for the waveguides themselves. Very high requirements are therefore imposed on their dimensional accuracy, which means that the corresponding production method requires complicated control processes and control units.
In the simplest case, the outside diameter of the component drawn from the blank is measured continuously by a diameter measuring device. The measurement values thus obtained are used to monitor the dimensional accuracy of the outside diameter and are simultaneously sent to a control unit, which determines deviation between the actual outside diameter and the nominal value and converts the deviations into changes in the drawing rate. In this case, the so-called "controlled variable" is the outside diameter, and the so-called "controlling variable" is the drawing rate. By means of a simple control system such as this, however, it is impossible to satisfy the high requirements imposed on the dimensional accuracy of the components. One of the main reasons for this is to be found in the "dead time" inherent in the measuring process. This is based on the fact that the outside diameter can be measured only after a certain period of time has passed after its actual formation. The effects of changes in the drawing parameters on the outside diameter are therefore evident only after a certain delay, namely, after the delay resulting from the measurement by the diameter measuring device.
To compensate for this measurement dead time of the process control system, a control concept known as the "Smith predictor" is proposed for a method of the general type in question in U.S. Pat. No. 5,314,517. According to this method, the outside diameter of an optical fiber is measured continuously in a contactless manner, and an imaginary outside diameter is predicted on the basis of the actual measurement values and the drawing rate by means of a predefined model of the process. The predicted outside diameter is then used as the controlled variable instead of the actually measured outside diameter.
To control a process by means of the Smith predictor, it is necessary to have a very accurate model of the process and to have exact knowledge of all the relevant process parameters as well as the dead measurement time. The time-variant, nonlinear, dynamic deformation behavior of the drawing process, however, cannot be easily predicted and therefore cannot be described accurately enough by means of a process model. In addition, the product parameters and dead times change unpredictably during the course of the process, which means that, in the known production method, fluctuations in the outside diameter of the fiber are unavoidable.
The invention pertains to a method for producing an elongated component of glass by drawing from a blank, in which the blank is sent to a heating zone, where it is softened region by region; where the component is drawn off continuously in a controlled manner from the softened region, which forms a drawing bulb; where at least one controlled variable of the process control system is measured continuously; and where the deviation between nominal and actual values of the controlled variable are determined from the measurement values thus obtained and converted to a change in a controlling variable. A process of the type described above is also known from U.S. Pat. No. 5,314,517. It has been found that the known control method is inadequate, especially in cases where sudden disturbances occur in the course of the process or in cases where changes occur in the process parameters during the course of the drawing process.
The invention is therefore to this extent based on the task of producing a component with high dimensional accuracy.
The invention also pertains to an apparatus for the production of an elongated component of glass by drawing from a blank, with a feed device for supplying the blank to a heating device; with a take-off device for drawing the component from a region of the blank which has been softened in the heating device; and with a measuring device for detecting a geometric variable of the component drawn from the blank, this measuring device being connected to a control unit, which determines the nominal/actual deviation of a controlled variable and sets a controlling variable as a function of the deviation.
An apparatus of the general type in question is also known from U.S. Pat. No. 5,314,517. In the known apparatus, a drawing tower for drawing an optical fiber from a preform is provided, which has a feed device for holding and feeding the preform continuously into a furnace in the vertical direction. The end of the preform projecting into the furnace is softened region by region, and a fiber is drawn from the softened region at a controllable drawing rate by a drawing device, the heating and drawing resulting in the formation of a drawing bulb. A device for measuring the outside diameter of the fiber by contactless optical means is installed between the furnace the drawing device. Both the diameter measuring device and the drawing device are connected to a control unit, which determines the deviation between the actual outside diameter of the fiber and the nominal value and converts this deviation into a change in the drawing rate. To compensate for the inherent dead time of the measurement, the control unit is provided with a Smith predictor.
The control concept of the known apparatus, which is based on a model of the process, is incapable of completely excluding fluctuations in the diameter of the optical waveguide.
The invention is therefore also based on the task of providing an apparatus by means of which elongated components of glass can be drawn with very high dimensional accuracy from a blank.